How does the optical heart rate on Spartan Wrist HR watches work?

Valencell is the leading innovator in wearable biometric data sensor technology.

In a nutshell, wrist heart rate technology measures heart rate via LEDs that track blood flow in your wrist.This means that you can see your heart rate data during a specific activity or in daily use without a chest heart rate strap.In practice, the optical heart rate sensor on the underside of the watch shines light into user's wrist using LEDs and measures the amount of light that is scattered by blood flow.

This is based on the fact that light entering the body will scatter in a predictable manner when the blood flow dynamics change, such as with changes in blood pulse rate or with changes in blood volume (cardiac output).